The term “shape-selective catalysis” describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by N. Y. Chen, W. E. Garwood and F. G. Dwyer, “Shape Selective Catalysis in Industrial Applications,” 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as isomerization, disproportionation, alkylation and transalkylation of aromatics are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational constraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective alkyl-substituted benzene disproportionation to para-dialkyl-substituted benzene.
A representative para-dialkyl-substituted benzene is para-xylene. The production of para-xylene is typically performed by methylation of toluene or by toluene disproportionation over a catalyst under conversion conditions. Examples include the reaction of toluene with methanol, as described by Chen et al., J. Amer. Chem. Soc. 101, 6783 (1979), and toluene disproportionation, as described by Pines in “The Chemistry of Catalytic Hydrocarbon Conversions” Academic Press, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture of the three xylene isomers, i.e., para-xylene, ortho-xylene, and meta-xylene. Depending upon the degree of selectivity of the catalyst for para-xylene (para-selectivity) and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of xylene produced as a proportion of the feedstock, is also affected by the catalyst and the reaction conditions.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts. One such method is to modify the catalyst by treatment with a “selectivating agent”. For example, U.S. Pat. Nos. 5,173,461, 4,950,835, 4,927,979, 4,465,886, 4,477,583, 4,379,761, 4,145,315, 4,127,616, 4,100,215, 4,090,981, 4,060,568, 5,476,823 and 3,698,157 disclose specific methods for contacting a catalyst with a selectivating agent containing silicon (“silicon compound”).
Various organic compounds have been employed as carriers for silicon to compounds in the silicon impregnation methods applied to zeolite catalysts. For example, U.S. Pat. Nos. 4,145,315, 4,127,616, 4,090,981 and 4,060,568 describe the use of inter alia C5-7 alkanes as solvents for silicon impregnation.
U.S. Pat. No. 4,548,914 describes another modification method involving impregnating catalysts with oxides that are difficult to reduce, such as those of magnesium, calcium, and/or phosphorus, followed by treatment with water vapor to improve para-selectivity.
European Patent No. 296,582 describes the modification of aluminosilicate catalysts by impregnating such catalysts with phosphorus-containing compounds and further modifying these catalysts by incorporating metals such as manganese, cobalt, silicon and Group IIA elements. The patent also describes the modification of zeolites with silicon compounds.
Traditionally, ex situ pre-selectivation of zeolites has involved single applications of the modifying compound. It may be noted, however, that the suggestion of multiple treatments was made in U.S. Pat. No. 4,283,306 to Herkes. The Herkes patent discloses the promotion of crystalline silica catalyst by application of an amorphous silica such as tetraethylorthosilicate. The Herkes patent contrasts the performance of catalyst treated once with an ethylorthosilicate solution followed by calcination against the performance of catalyst treated twice with ethylorthosilicate and calcined after each treatment. The Herkes disclosure shows that the twice-treated catalyst is less active and less selective than the once-treated catalyst as measured by methylation of toluene by methanol, indicating that multiple ex situ selectivation confers no benefit and in fact reduces a catalyst's efficacy in shape-selective reactions.
Steaming has also been used in the preparation of zeolite catalysts to modify the alpha or improve stability. For example, U.S. Pat. No. 4,559,314 describes steaming a zeolite/binder composite at 200-500° C. for at least an hour to enhance activity by raising the alpha. U.S. Pat. No. 4,522,929 describes pre-steaming a fresh zeolite catalyst so that the alpha activity first rises then falls to the level of the fresh unsteamed catalyst, producing a stable catalyst which may be used in xylene isomerization. U.S. Pat. No. 4,443,554 describes steaming inactive zeolites (Na ZSM-5) to increase alpha activity. U.S. Pat. No. 4,487,843 describes contacting a zeolite with steam prior to loading with a Group IIIB metal.
However, a need still exists for novel catalysts to achieve disproportionation of aromatic compounds to provide, for example, para-xylene in good yields and/or selectivity.